The present invention relates to a cooling system for a motor and to a cooling control method.
An electric vehicle, including a hybrid vehicle, has a system in which power is supplied to a driving motor from a battery via a power converter, and a forced cooling means is provided for suppressing a temperature rise of the power converter and driving motor due to heat generated in correspondence with the operations of the power converter and driving motor.
The forced cooling means is structured so as to force feed a refrigerant, such as fresh air or a cooling liquid (an antifreezing solution), to the power converter and driving motor when the temperatures of the power converter and driving motor rise up to a predetermined cooling start temperature, thereby force cooling them.
For example, a patent document 1 (Japanese Application Patent Laid-open Publication No. Hei 07-213091) and a patent document 2 (Japanese Application Patent Laid-open Publication No. Hei 08-33104) disclose a cooling device for controlling the cooling air speed according to the temperature of heat radiating fins of a semiconductor element of a power converter, which is used for controlling an electric vehicle motor.
Further, a patent document 3 (Japanese Application Patent Laid-open Publication No. Hei 10-210790) discloses an inverter cooling device for an electric vehicle for detecting the temperature of a semiconductor element of an which is used for supplying a current to a motor and for controlling the flow rate of a refrigerant according to the temperature of the semiconductor element and the change rate thereof.
Further, a patent document 4 (Japanese Application Patent Laid-open Announcement Publication No. 2001-527612) discloses a cooling device for detecting the temperatures of a temperature control fluid and ambient air so as to control the temperature of the engine oil of a vehicle at a proper temperature.
For a driving motor of a driving device for an electric vehicle, a DC commutator motor or an inverter driving type DC non-commutator motor is generally used, and the power supply to such a driving motor is controlled by a power converter, such as a chopper circuit or an inverter circuit. During the operation (power supply control), in the driving motor, a loss due to the flow of a current through a coil or a mechanical loss during high-speed rotation is caused; and, also, in the power converter, a loss is caused during power supply to a semiconductor element for power conversion control or at the time of switching. These losses are finally converted to heat, and the total amount of heat reaches several kW at the maximum.
Such generation of heat causes a temperature rise of the driving motor and power converter; and, when it is left as it is, the driving motor and power converter cannot exhibit a predetermined performance due to the temperature rise. Furthermore, the insulating material is reduced in withstand voltage and is finally destroyed, so that the generated heat must be removed.
As a forced cooling means, which effectively radiates a large amount of generated heat and which can be mounted in a limited space, it is common to use a method which employs a forced flow of a refrigerant, using a device such as a pump or a fan, and which causes heat to be radiated by heat exchange between a device generating heat and the refrigerant.
The forced cooling control using a pump or a fan is structured so as to detect the temperatures of the driving motor and power converter, compare them with a cooling start temperature which is set fixedly, and start the operation of the pump or fan when the detected temperatures reach the set cooling start temperature.
Under this forced cooling control method, the cooling start temperature is fixed, so that in winter, when the air temperature is low, the difference between the temperature at the time of operation start of a driving device for an electric vehicle and the maximum temperature during operation is large.
In the power converter, when a temperature cycle is added to the semiconductor element for power conversion control that is used for power conversion, thermal stress caused by the difference in the linear expansion coefficient between the members is generated, and a thermal fatigue failure is generated. Therefore, to avoid generation of a failure due to thermal stress, it is desirable to avoid an excessive temperature difference in the temperature cycle. Moreover, it is required to force cool the semiconductor element for power conversion control so as to maintain it within the heat resistance allowable temperature range before high-temperature failure occurs, or limit the amount of heat.
Further, in the driving motor, since the dielectric strength of the electrical parts and the magnetic characteristics of magnetic parts are reduced in correspondence with the temperature rise, it is desirable to cool these parts so as to prevent the temperature of each of them from exceeding the heat resistance allowable temperature or limit the amount of heat.
Furthermore, when a device such as a pump or a fan is operated, energy consumption follows, so that when such a device is activated often, the energy consumption is increased and the energy consumption rate of the vehicle gets worse.
This is a problem common to not only a cooling system for a driving device for an electric vehicle, but also for various motors using a driving motor.